These compounds are synthesis intermediates used for the production of fluorinated surfactants and of products for the hydrophobic and oleophobic treatment of substrates such as textiles and leathers.
These thiols have already been described and are generally obtained by reacting a 2-(perfluoroalkyl)ethyl iodide R.sub.f C.sub.2 H.sub.4 I with thiourea, followed by the alkaline hydrolysis of the thiouronium salt thus obtained: ##STR1##
This preparation process is the use of a method which is conventional for converting alkyl iodides and bromides into the corresponding thiol. For non-fluorinated thiols, this method has widely been described, mainly starting with alkyl bromides, which are more readily available than the iodides. The condensation of the halide with thiourea is generally carried out in an alcoholic or aqueous-alcoholic medium to achieve a homogeneous reaction medium. See E. Reid, Organic Chemistry of Bivalent Sulfur, Vol. I., page 32-35, Chemical Publishing Co., New York, 1958, and Organic Syntheses, coll. Vol. 3, page 363, hereby incorporated by reference.
Many patents describe the preparation, according to the same process, of 2-(perfluoroalkyl)ethanethiols R.sub.f C.sub.2 H.sub.4 SH. There may be mentioned, for example, U.S. Pat. Nos. 3,088,849; 3,544,663; and 3,172,910; as well as French Pat. Nos. 2,083,422; and 2,113,219. All these documents present examples for the preparation of these 2-(perfluoroalkyl)ethanethiols by decomposing the isothiouronium salts obtained by reacting 2-(perfluoroalkyl)ethyl iodides R.sub.f C.sub.2 H.sub.4 I with thiourea. In all these examples, this reaction is always carried out in an alcoholic (methanol, ethanol, isopropanol or butanol) medium, probably with a view to being able to solubilize the two reagents as well as the isothiouronium salt.
Although 2-(perfluoroalkyl)ethanethiols, especially those containing a relatively long R.sub.f chain (for example C.sub.6 F.sub.13) only have a weak odor of mercaptans, this method of preparation in an alcoholic medium always leads unfortunately to the formation of foul-smelling by-products. These make the process virtually unuseable on an industrial scale ina conventional plant. Essentially, foul-smelling by-products are formed during the second reaction stage, i.e., during the alkaline decomposition of the isothiouronium salt.